Several metabolic and morphologic changes have been claimed to precede the onset of diabetes, including changes in the pattern and quantity of insulin secretion and alteration in the thickness of capillary basement membranes. This study will determine if muscle capillary basement membrane thickening is a characteristic of the prediabetic state, and if so whether the thickening is present many years before the onset of diabetes, and therefore can be considered a prediabetic marker, or whether it develops pari passu with metabolic abnormalities that occur prior to the onset of diabetic hyperglycemia. Pima Indians with two diabetic parents, and with neither patient diabetic received oral and intravenous glucose tolerance tests, and a biopsy of the quadriceps muscle from which quantitative determinations of the thickness of the capillary basement membrane with increasing age in those with diabetic parents compared to those without. The results will help to determine if vascular lesions at the level of the capillary are present before hyperglycemia develops.